Subsurface cracks

ABSTRACT

Pieces of flat glass are cut to desired size without the necessity of grinding to size and polishing. Edges of the piece are cut in accordance with a procedure involving the use of a large-diameter scoring wheel at a greater-than-usual applied pressure, to produce a subsurface crack that is substantially free of serrations, followed by the application of a bending moment about the crack to propagate a fracture in the piece of glass. Light seaming of the top and bottom portions of the edges completes the preparation of these edges.

United States Patent 11 1 Ernsberger et al.

[ SUBSURFACE CRACKS [75] Inventors: Fred M. Ernsberger, Pittsburgh,

Charles M. Hullabaugh, Allison Park, both of Pa. [73] Assignee: PPGIndustries, Inc., Pittsburgh,

Pa. [22] Filed: Dec. 6, I973 211 Appl. No.: 422,393

Related U.S. Application Data [62] Division of Ser. No. 242,5ll, April10, I972.

[52] U.S. Cl. 225/2 [51] Int. Cl B26f 3/00 [58] Field of Search 225/1,2, I03, 96.5, 93.5; 30/l64.95

[56] References Cited UNITED STATES PATENTS 2,707,849 5/1955 DeVore30/l64.95 3,474,944 10/1969 Chatelain et al 225/103 X 1 Feb. 11, 1975FOREIGN PATENTS OR APPLICATIONS 770,316 1/l972 Belgium PrimaryExaminer-Andrew R. Jihasz Assistant Examiner-Leon Gilden Attorney,Agent, or Firm-Thomas F. Shanahan [57] ABSTRACT Pieces of flat glass arecut to desired size without the necessity of grinding to size andpolishing. Edges of the piece are cut in accordance with a procedureinvolving the use of a large-diameter scoring wheel at agreater-than-usual applied pressure, to produce a subsurface crack thatis substantially free of serrations, followed by the application of abending moment about the crack to propagate a fracture in the piece ofglass. Light seaming of the top and bottom portions of the edgescompletes the preparation of these edges.

4 Claims, 13 Drawing Figures 1 SUBSURFACE CRACKS This is a division, ofapplication Ser. No. 242,511, filed Apr. I0, 1972.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to glass articles and to a method and an apparatus for producingsaid glass articles, and in particular, for the manufacture ofarchitectural panels, furniture tops and other relatively thick glassarticles, for example, in excess of IO millimeters (especially in therange of 18 to 36 millimeters or above), having dimensions such as 4meters by 8 me ters.

2. Description of the Prior Art:

In the manufacture of architectural-glass panels and furniture tops ofthe kind indicated above, it has hitherto beeircommon to obtain panelsof a desired size by hand scoring and mechanical snapping of the edgesof the glass to yield a piece somewhat greater in its dimensions thanthe final size desired, followed by finishing operations, such as thegrinding of the cut edges of the piece to the desired size and thepolishing of the ground edges. The grinding and polishing aretime-consuming and costly operations, but they have hitherto beenconsidered necessary, particularly in cutting glass sheets ofsubstantial thickness.

It is important that architectural panels exhibit adequate edgestrength. When tested in accordance with the conventional beam-loadingtest, the ground-andpolished edges of a 4- by 8 meter sheet,approximately 18 millimeters thick, produced by a process including thesteps of normal scoring, snapping, grinding and polishing, exhibitstrength values such as about 4.6 to 4.9 kilograms per squarecentimeter. Panels exhibiting values substantially lower than about 4kilograms per square centimeter are noticeably more susceptible tobreakage.

U. S. application Ser. No. 57,574, filed July 23, 1970, and U.S.application Ser. No. 68,735, filed Sept. 1, 1970, both by Robert P.DeTorre, disclose a trimming procedure that involves the application ofa surface deep score under relatively high pressure by a largediameter,blunt scoring wheel, followed by the propagation of the score into afracture and a light seaming operation on the top and bottom portions ofthe edges of the glass so cut.

SUMMARY OF THE INVENTION According to the present invention, a glassarticle is produced having a top surface, a bottom surface and a cutedge extending therebetwee-n in a direction substantially perpendicularto the top surface and the bottom surface. The cut edge contains twomarkings, each of which is substantially parallel to the top and bottomsurfaces. The area between the markings is substantially free ofserrations. The markings are adjacent to one of the surfaces and areindicative of the extent of a subsurface crack that was placed in theglass article to cut the article along the edge. Light seaming removessharp corners between the edge and each of the major surfaces and italso removes the markings.

As used in this application, the terms subsurface crack or subsurfacescore refer to a discontinuity such as a crack or score, respectively,that is within the thickness of the piece of glass and does not extendto a major surface of the piece. The term subsurface crack refers to adiscontinuity in the piece of glass without substantial serrations. Theterm subsurface score" refers to a discontinuity in the piece of glasswith serrations. The term "subsurface discontinuity includes subsurfacecracks" and subsurface scores."

In accordance with the present invention, a glass article is produced bya process and an apparatus which avoids the use of grinding to size andedge polishing. The process includes the imposition of a subsurfacecrack along an intended path of cut into the piece of glass and theprojection of the crack deeper into the thickness of said piece by aseparate, non-simultaneous step, such as the application of a bendingmoment about the crack. Prior to the creation of the crack, a surfacedefect is placed in the piece of glass on the intended path of cut at alocation where the crack is to initiate. The apparatus for producing thesubsurface crack consists of a large-diameter scoring wheel, such as,for example, at least approximately l2 millimeters, and preferablybetween approximately 19 and millimeters, in diameter, having a bluntcutting angle, such as, for example, between approximately and 'at highforces, such as, for example, approximately 80 kilograms toapproximately 460 kilograms, and even greater. This is at least two tothree times the forces used in surface deep scoring. Relatively lightseaming of the top and bottom portions of the cut edge yields cut edgesthat are substantially as strong as conventional ground-and-polishededges and edges produced by a surface deep-scoring process. In addition,less seaming is required in a cut edge that is produced using asubsurface crack than in an edge that is produced by a surface deepscore, due to the absence ofwing in subsurface cracks.

Accordingly, it is an object of the present invention to produce cutedges that are smooth, strong, straight, pristine and perpendicular tothe major surfaces of the piece of glass.

It is a further object of the present invention to produce such edgeswhile avoiding the use of grinding and polishing.

It is a further object of the present invention to provide a method andapparatus for cutting all glass, and in particular, glass in excess ofapproximately l0 millimeters in thickness by a relatively uncomplicatedprocedure that involves the application of a subsurface crack, followedby a bending moment about the crack and a small amount of seaming.

It is a further object of the present invention to produce a cut edgethat is at least equal in quality to those produced by surface deepscoring, with a lesser amount of seaming.

It is a further object of the present invention to find a practicalmeans for generating a continuous crack that will weaken a sheet of flatglass to the extent necessary so that it may be severed withoutincurring surface crushing or edge damage.

It is a further object of the present invention to produce a cut edge atrelatively high speeds, such as approximately 2 meters per second.

DESCRIPTION OF THE DRAWINGS A complete understanding of the followinginvention may be obtained from the foregoing and following descriptionthereof, taken together with the appended drawings, which are not drawnto scale unless noted, and in which:

FIG. 1 is a diagrammatic view of a scoring apparatus applying asubsurface discontinuity to a piece of flat glass;

FIG. 2 is a vertical cross-sectional view of a cutting wheel used toproduce subsurface scores;

FIG. 2A is a vertical cross-sectional view of a cutting wheel used toproduce subsurface cracks according to the present invention; FIG. 3 isan enlarged end view of the subsurface discontinuity;

FIG. 4 is an enlarged end view of a surface deep score;

FIG. 5 is an elevation view of a snapping apparatus in position to applya bending moment about the subsurface discontinuity;

FIG. 6 is an elevation view of an edge of a piece of glass cut inaccordance with a procedure that utilizes a wheel with a relativelylarge amount of friction between the wheel and its holder;

FIG. 7 is an enlarged view of an upper portion of the cut edge shown inFIG. 6;

FIG. 8 is an elevation view of an edge of a piece of glass cut inaccordance with surface deep-scoring techniques;

FIG. 9 is an enlarged view of an upper portion of the cut edge shown inFIG. 8;

FIG. I0 is a diagrammatic view of a piece of glass with a subsurfacescore made by a cutting wheel with a relatively large amount of frictionbetween the wheel and its holder;

FIG. 11 is an elevation view of an edge of a piece of glass cut inaccordance with the present invention, using a wheel with a relativelysmall amount of friction between the wheel and its holder; and

FIG. 12 is a schematic view of an edge or corner portion of a piece ofglass being seamed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, anapparatus 12 is shown applying a subsurface discontinuity 10, such as asubsurface crack or score, to a central portion of a piece of glass Galong an intended path of cut and in a direction 15 that issubstantially parallel to top surface 37 and bottom surface 76, whilethe glass is supported on a table T. At least a substantial portion ofdiscontinuity is spaced from top surface 37 and bottom surface 76 alongthe length of discontinuity I0. Apparatus 12, including a scoring orcutting wheel 14, is shown moving from one end of glass G to the otherend in the direction of arrow to apply the subsurface discontinuity 10.Wheel 14 may have a central axle hole 24, such as high friction wheel 14in FIG. 2, or, in accordance with the present invention, it may have anintegral shaft 24', such as low friction wheel 14" in FIG. 2A.

One skilled in the art will appreciate that there are many commerciallyavailable devices for housing scoring wheel 14. It is well known, forexample, to supply the necessary scoring pressure to a cutting wheel bymeans of a fluid pressure such as air or hydraulic fluid. Further, U.S.application Ser. No. 128,384, filed on Mar. 26, I971, by David A. Bier,suggests that a cutting wheel may be actuated by a constant-reluctancemotor means. Any suitable means may be used to supply the load to wheel14.

A permanent indentation 11 is created in top surface 37 by the wheel 14directly above discontinuity 10. Indentation 11 may be approximately0.00l millimeter deep and approximately 0.015 millimeter wide. There aretwo theories relating to the creation of indentation 11. The firsttheory is that there is plastic flow of the glass from one area toanother without any change in total volume of the glass. The secondtheory is that the glass is compressed or densified with the totalvolume of the glass being reduced. Theoretically, if the glass isdensified (second theory), the index of refraction of the glass will bealtered, but if the glass flows plastically (first theory), the index ofrefraction will not be altered. Experimental results indicate that theindex of refraction, immediately beneath indentation II, isapproximately 5 percerit higher than that of normal glass. This tends tofavor the theory that densification occurs beneath the indentation 11,but it does not eliminate the possibility that there is a small amountof plastic flow in combination with densification. However, plastic flowrequires deformation in shear, which necessarily involves the breakingof interatomic bonds, and those skilled in the art will recognize thatthis phenomenon cannot occur in a covalent material, such as glass,unless the material is strained at a temperature from one-half totwothirds of its melting point. In the present case, there is no reasonto suspect that the glass is heated to such a degree. In addition, wheredensification occurs in the glass, tensile stresses are created toassist in severing the glass.

Referring to FIG. 2, there is shown a detailed view of a high frictioncutting or scoring wheel 14 that may be used to produce a subsurfacescore. Wheel 14' is made of tungsten carbide or other suitable materialof hardness of about 7 or more on Mohs scale and having a radius inexcess of approximately 6 millimeters, and preferably withinapproximately 9 to approximately 50 millimeters. The base angle, i.e.,the angle between the two sides 16 and 18, if extended, is about l20,and the angle between the sides 20 and 22 (hereafter referred to as thecutting angle) is between approximately and approximately 170, withapproximately providing best subsurface scores. With cutting angles lessthan approximately 150, defects such as spall and wing may occur. Theterm spall may be defined as a chip or flake out of the edge of thepiece of glass. The term wing may be defined as a lateral crack oneither side of a score line, projected outward under the glass surfaceby the action of a scoring tool. With cutting angles betweenapproximately 150 and approximately 155, surface deep scores aregenerally produced. If the cutting angle is greater than approximatelyit is extremely difficult to produce any discontinuity beneath the apex27 of the wheel 14. If pressure is applied to a wheel 14 having acutting angle greater than approximately 170, until the glass fails, thefailure will generally occur adjacent to the point where side 16 meetssurface 20, or side 18 meets surface 22. This is probably due to thefact that cutting angles in excess of approximately 170 merely place theglass in compression along the entire width of surfaces 20 and 22.

The wheel 14' is shown with a central axle hole 24 which functions as ameans for rotatably mounting said wheel on a shaft that is passedthrough the axle hole 24. Hole 24 may be, for example, 2.4 millimetersin diameter. With such a setup, there may be a relatively large amountof friction between the cutting wheel and its holder, and for thisreason, thistype of wheel is referred to herein as a high frictionwheel."

Referring to FIG. 2A, there is shown a low friction wheel 14" forproducing subsurface cracks in accordance with the present invention.Wheel 14" is identical to wheel 14' except that wheel 14 'has anintegral shaft 24 instead of a hole 24. The shaft 24 may be mounted inbearings, such as ball bearings 29, to minimize, or even eliminate,friction between the wheel and its holder. For this reason, this type ofwheel is referred to herein as a low friction wheel." As discussedhereinbelow, a low friction wheel has more of a tendency to roll thandoes a high friction wheel. This reduces the tensile stressesimmediately behind the wheel to minimize, or even eliminate, unwantedcircular fractures.

Wheel 14" may be, for example, 29 millimeters in diameter and urged intocontact with a piece of glass G that is approximately 19 millimetersthick at a force of approximately 175 kilograms, to produce a subsurfacecrack that starts approximately 0.01 millimeter from the top surface 37of glass G and extends for approximately 2 to 2.5 millimeters into thethickness of glass G. Cracks so produced correspond to the intendedlocation of the edge of the finished piece. To guide apparatus 12, astraight-edge member may be secured to the glass G as is conventional inprior-art scor- Although a preferred embodiment of the present inventionincorporates a cutting wheel or disc, other means will become apparentto carry out the present invention. For example, one may wish toconstruct a member that comprises a continuous chain forming a curvedcutting edge rather than a wheel. It would still be necessary tomaintain both the blunt cutting angles and the high pressures describedherein. It is also necessary to maintain the effective radius of thecontinuous chain within the above-mentioned range. For example,continuous chain could take the path of an oval, but the radius of theoval at the point of contact with the chain and the glass (effectiveradius) should be within the same range as the radius (or effectiveradius) of a cutting wheel.

It is important to note the importance of orienting wheel 14 such thatit is substantially perpendicular to the surface of the glass to be cut.A subsurface discontinuity generally extends in the same direction asthe cutting wheel. Therefore, if the cutting wheel is not perpendicularto the glass surface, the resultant subsurface discontinuity will not beperpendicular. Referring to FIGS. 2 and 2A, angles A and B indicate theangles between the cutting wheel and the glass surface. With a cuttingwheel having a cutting angle of 165, it is preferred that angle A andangle 8 be maintained at 75.

Referring to FIG. 3, there is shown a partial view, greatly magnified,of a surface 35 that is formed when the piece of glass G has beensevered along dashed line 36 in FIG. 1 by running a cut. It should beunderstood that in a normal cutting operation, the glass is not severedalong line 36. This is only done to illustrate a means for detecting asubsurface discontinuity. Subsurface discontinuity is located directlybeneath the path of ,wheel 14, starting, for example, at approximately0.01 millimeter below the top major surface 37 of glass G and extendingin a direction that is substantially perpendicular to surface 37 forapproximately 2.5 millimeters. Mark 38 is peculiar to the inner sectionof a severed surface 35 with a subsurface discontinuity. It should beunderstood that mark 38 is not a crack, but merely a slight ridge,caused by a fracture propagation from two different locations. The termWallner lines" is used in the art to describe lines on a severed surfacethat indicate the direction of fracture propagation as a cut is run.Wallner lines 40, 42, 42', 44, 44 and 46 indicate that discontinuity 10does not extend entirely to the top major surface 37 of the piece ofglass G, as described more fully hereinbelow.

FIG. 4 is a view similar to FIG. 3 showing how a sev' ered surface 35would look if a surface score 10' were placed in a piece of glass G. andthe piece were then severed by running a out along a plane that isperpendicular to score 10'. Wallner lines similar to those shown at 50,52, 54 and 56 will extend across surface 35. There is no mark (such asmark 38 in FIG. 3). and this indicates that the severed surface includesa surface score. Note that Wallner lines 50, 52, 54 and 56 bow towardthe bottom right of glass G'and the top portion of each Wallner line isfarther to the right than the bottom portion. This indicates that afracture was run from left to right and from top to bottom by a bendingmoment about the top major surface 37 ofthe glass G to place it intension.

It is apparent that the Wallner line pattern in FIG. 4 is significantlydifferent from the Wallner line pattern in FIG. 3. In FIG. 4, thefracture propagation starts at 50 and moves from left to right. Thepattern is similar in FIG. 3 at the start of fracture propagation, asevidenced by Wallner line 40. When the propagation in FIG. 3 reachessubsurface discontinuity 10, the original single Wallner line splitsinto two independent lines 42 and 42. This is because some of thepropagation occurs between the top major surface 37 of the piece ofglass G and the uppermost portion of subsurface discontinuity l0, andsome of the propagation occurs between the bottommost portion ofsubsurface discontinuity l0 and the bottom major surface (not shown inFIG. 3) of the piece of glass G, as illustrated by Wallner lines 42 and42', respectively. After both fronts have traveled around the subsurfacediscontinuity, they approach each other, as at 44 and 44', meet at mark38, and merge to form a single front, as indicated by Wallner line 46.Wallner line 44 is in a plane that is slightly offset from a plane inwhich Wallner line 44' is located. As a result, where Wallner line 44meets Wallner line 44', there is a slight protrusion which has beenshown as mark 38. By the time the fronts have progressed to Wallner line46, they have merged into a single front in a common plane. In contrast.the Wallner lines in FIG. 4 do not split at score 10', because score 10'contacts the surface 37' of the glass.

Experience indicates that mark 38 is always present in a subsurfacediscontinuity, such as a score or crack, pointing in the direction offracture propagation. This provides a means for establishing whether ornot a discontinuity is a subsurface discontinuity or a surfacediscontinuity. It also provides a method for establishing the directionof fracture propagation where one is severing in a plane that intersectsa subsurface discontinuity.

Referring to FIG. 5, there is shown an elevation view of a snappingapparatus 60 in position to apply a bending moment about subsurfacediscontinuity 10. The apparatus may consist of two top anvils 62 and 64,and a bottom anvil 66. Glass G may be placed upon a table so that aportion of the subsurface discontinuity l0 overlaps the table. A bendingmoment may be applied at the end of the piece of glass G that overlapsthe table to run a cut along the subsurface discontinuity 10. It issometimes difficult, especially with pieces of glass that are relativelylong and thick (such as l9-millimeter thick glass in excess of 3 metersin length), to run a cut in the manner described. Under suchcircumstances, a narrow member or plate, approximately l2 millimeters inwidth, may be placed between the glass and the table, directlybeneath-the subsurface discontinuity 10. This places the top surface ofthe piece of glass in tension along the discontinuity and reduces theenergy necessary to run a cut along the entire length of the piece. Cutedges are produced that are smooth, strong, straight, pristine andperpendicular to the major surfaces of the piece.

After the glass has been snapped, there may be conducted an inspectionto determine the quality of the cut that has been opened. In theinspection along the cut edge, looking perpendicularly to the cut edge,it is customary to see a pattern such as that indicated in FIG. 6, whena high friction wheel 14 has been used. The top surface of the glass isthere designated with the numeral 37 and the bottom designated with thenumeral 76. A short distance below top surface 37 is seen a marking 72and a marking 74 which indicate the extent of the subsurface scorecaused by the cutting wheel 14'. The marking 72 is generallyapproximately 0.01 millimeter from the top surface of the glass (thishas been exaggerated in FIGS. 6 and 7), and the marking 74 may beapproximately 0.5 to 4 millimeters from the marking 72, or even more.FIG. 7 is a magnified view of an upper portion of the cut edge shown inFIG. 6, further illustrating markings 72 and 74 and showing theserrations 73 therebetween. Note that each serration 73 approximates aquarter of a circle and markings 72 and 74 each approximate a straightline that is parallel to top surface 37 and bottom surface 76.

Referring to FIG. 8, there is shown a cut edge of the piece of glass Gthat was severed with a surface deep score. The piece of glass G' has atop surface 37' and a bottom surface 76. A surface deep score 10 extendsfrom top surface 37 to marking 74. FIG. 9 is a magnified view of anupper portion of the cut edge in FIG. 8, further illustrating marking 74and showing serrations 73'. Each serration 73' approximates asemi-circle. Note that marking 74 is a substantially straight line whilemarking 74' is jagged. This is significant since it is often necessaryto do additional seaming to remove some of the longer points frommarking 74. Additionally, with surface deep scoring, the serrationsprotrude from the glass by approximately 0.25 to 0.5 millimeter, whilethe serrations in subsurface scores are only about one-half of thatamount, or approximately 0.125 to 0.25 millimeter. Finally, as describedmore fully hereinbelow, long wings develop in surface deep scoring whenextremely large-diameter wheels are used, but they do not develop insubsurface scoring. For these reasons, subsurface deep scores requiresubstantially less seaming than do surface deep scores.

It should be emphasized that with wheel 14' there is a relatively largefrictional force between the wheel and its holder. Using a wheel with anintegral shaft 24 and ball bearings 29, such as wheel 14'' in FIG. 2A,eliminates most of the frictional force between the wheel and itsholder. This produces a subsurface crack that is of even higher qualitythan a subsurface score, since serrations are substantially eliminated.

Referring to FIG. 10, there is shown a view of a subsurface score 10"made with a high friction wheel 14' in a piece of glass G. Indentation11 has been omitted, and score 10" and defects have been exaggerated forthe sake of clarity. These defects are caused by the high frictionbetween the wheel 14' and its holder. With the higher friction, there isa tendency for the wheel to slide rather than roll. This increases thecompressive stresses immediately in front of the wheel and the tensilestresses immediately after the wheel. The increased tensile stresscauses small circular fractures, such as defects 70, to be formed. Witha wheel, such as wheel 14", having a low frictional force between itselfand its holder, the tendency for the wheel to slide is minimized. Thisreduces the tensile stress behind the wheel and defects 70 do notappear. If the speed of advance of the low friction wheel is increased,for example, to at least approximately l meter per second for wheelshaving a diameter of 12.7 millimeters, or 2 meters per second for wheelshaving a diameter of I00 millimeters, or if forces in excess of thoselisted in Table B are used, it is likely that a surface score willresult from the low friction wheel.

Referring to FIG. 11, there is shown a cut edge of a piece of glass Gthat was severed with a subsurface crack, using a low friction wheel,such as wheel 14''. It should be noted that a surface defect, such as ahand nick, should be placed in the glass along the intended path of cutbefore the subsurface crack is initiated. This functions as a startingpoint for the subsurface crack.

The edge illustrated in FIG. 11 is similar to the one shown in FIG. 6,the only difference being that area 73' is smooth, with little or noserrations between marking 72" and marking 74" in FIG. 11. This isbecause it is the circular defects 70 appear to cause the serrations,and when circular defects 70 are eliminated, the serrations are alsoeliminated.

The low friction wheel 14" would appear to be more desirable than thehigh friction wheel 14, since the low friction wheel produces a cut edgewith no serrations at a speed approximately four times that of a highfriction wheel. However, it should be noted that all subsurface scoresand cracks must be initiated ata surface of the glass. With a subsurfacescore made with high friction wheel 14, defects 70 function as thestarting point for the score. When there are no defects 70, such as witha subsurface crack, it is necessary to place a surface defect in theglass to initiate the subsurface crack. Since the defects 70 with a highfriction wheel may be kept small (and easily removed with lightseaming), this type of wheel is preferred unless high scoring speeds arenecessary, such as in the on-line" scoring of a continuous ribbon ofglass.

The fact that serrations 73' protrude from the edge of the piece ofglass in FIG. 8 about twice the amount of serrations 73 in FIG. 6 makesthe edge shown in FIG. 6 more desirable than the edge shown in FIG. 8.Further, the lack of serrations 73" makes the edge in FIG. ll'even moredesirable. The fact that cut edges shown in FIGS. 6 and 11 were madewith subsurface discontinuities is significant in itself, since cuttingoil and healing are eliminated with subsurface discontinuities. But ifone obtains a discontinuity having the physical characteristics of thescore shown in FIG. 6 or of the crack shown in FIG. 11, except that thediscontinuity intersects the surface of the piece, it is stillpreferable to the score shown in FIG. 8. It is important that a score orcrack be of suitable quality such that a fracture may be propagated withlittle or no edge damage to the piece of glass so that seaming may beminimized. The edges shown in FIGS. 6, 8 and 11 are all of such quality,but serrations 73 require even less seaming than serrations 73, and area73" requires even less seaming than serrations 73.

With control of various parameters, such as wheel diameter, cuttingangle, force applied to the wheel, etc., it is possible to produce acrack or a score that is beneath the major surfaces of the glass. itshould be kept in mind, however, that there may be situations where acrack or a score contacts a major surface of the piece of glass, butretains the physical characteristics of a subsurface crack or score,respectively.

U.S. application Ser. No. 68,305, filed on Aug. 31, 1970, by David A.Bier et al. discloses that in the cutting of blanks, such asWindshields, it is advantageous to increase the depth of a score at thecorners of the blank. In such a case, subsurface cracks may be used tooutline the entire blank except for the corners where a parameter suchas speed may be changed to yield a surface score, which weakens theglass to a greater extent, at the corners.

The inspection further comprises viewing the glass vertically, i.e., ina direction perpendicular to the major surfaces of the sheet of glass,to detect wing or undercut defects. A satisfactory cut exhibits no suchdefects, or, at the worst, ones so minor as to be removed during asubsequent seaming operation.

As used in this application, the term subsurface score refers to thearea between marking 72 and marking 74 in FIGS. 6 and 7. The termsubsurface crack refers to the area between marking 72 and marking 74"in FIG. 11. The term fracture refers to the area marking 74 and thebottom surface 76 in FIG. 6, or a similar area in FIG. 11.

As a final step in the process of the present invention, there isconducted a finishing, such as light seaming, of only the upper andbottom portions of the edges of the piece of glass so cut. This leaves asmooth edge with no evidence ofmarkings 72" and 74". There may be used,as illustrated for example, in FIG. 12 a hand belt sander using a belt75 millimeters wide by 600 millimeters long. This is a conventionaloperation, and it does not require further elaboration or explanation.

The result is that there is produced a finished piece of glass thatcompares favorably in its edge strength to similar pieces produced bythe prior-art method of rough cutting, mechanical snapping, grinding tosize, and then polishing. The pieces of the present invention have edgestrengths of about 4.4 to about 4.7 kilograms per square centimeter inthe conventional beamloading test, in comparison with strengths such as4.6 to 4.9 kilograms per square centimeter for the prior-artground-and-polished pieces. Either will meet specifications on customaryglazing installations. In achieving the edge-strength values indicatedabove, the final limited seaming operation is important. Without thefinal seaming operation, the edge strength is on the order of 3.8 to 4.0kilograms per square centimeter.

As the glass becomes thicker, it becomes increasingly difficult toproduce with a cutting wheel of a given diameter a subsurfacediscontinuity of the required depth without causing a development ofwing. This means that with thicker glass, a larger cutting wheel shouldbe used, and with thinner glass, the use of a somewhat smaller cuttingwheel is permissible.

The present invention utilizes a subsurface crack in a process and anapparatus for cutting a glass article that is, in many circumstances,superior to any known in the prior art. First, in accordance with thepresent invention, serrations are substantially eliminated to minimizethe amount of seaming necessary to finish the edge. Second, the presenceof significant wing is eliminated. Third. the presence ofglass chipsthat have heretofore plagued cutting processes is minimized and almosteliminated. This eliminates the necessity of removing these chips andthe surface damage to the glass caused by the presence of the chips.Fourth, it has been common to use cutting oil to protect a score fromatmospheric moisture. With the present invention, since the subsurfacecrack does not come in contact with the atmosphere, there is no need toprotect it from atmospheric moisture and therefore no need to usecutting oil. This eliminates the problem of removing the cutting oilafter the discontinuity has been applied. Fifth, a subsurfacediscontinuity does not heal when left standing, as does a surface score.When a score heals, the stress produced by the scoring action disappearsand the cut is more difficult to open. The present invention allows oneto place a subsurface crack in the glass and store it for a period oftime before snapping it. Sixth, subsurface cracks may be produced inglass at relatively high speeds. Finally, due to the fact that there isno surface damage to the glass, the scoring wheel is subjected to lessof an abrasive action and wheel life is increased.

As is the case with surface scores, the depth of a subsurfacediscontinuity is directly related to the pressure applied to the scoringwheel. As pressure is increased, the depth of the subsurfacediscontinuity also increases. However, for a wheel of any givendiameter, there is a practical limit to the amount of pressure that canbe applied. If too much pressure is applied to the cutting wheel,excessive wing appears. By excessive or significant wing, it is meantthat a substantial amount of seaming (more than about 6 millimeters) isnecessary to remove the wing. For example, with surface scores, a normalcutting wheel of about 6 millimeters in diameter with a cutting angle ofl60 has a maximum score depth of about 1 millimeter in 12- millimeterglass at a force of about 18 kilograms. if the force is increased, acrude fracture and significant wing result without any increase in scoredepth. In order to increase the depth of the surface score, withoutproducing significant edge defects, it has been necessary to increasethe diameter of the wheel. As the diameter of the wheel is increased, itis possible to obtain a score of greater depth by increasing the forceapplied to the wheel. For example, a high friction cutting wheel havinga diameter of 19 millimeters, with a cutting angle of 160, will producea surface score depth of about 2.3 millimeters with a force of aboutkilograms, without significant surface defects, if the wheel is moved atmore than about 20 centimeters per second. If the force alone isincreased, the depth of the surface score will not increase, and surfacedefects and perhaps a crude fracture will result.

With a high friction cutting wheel having a diameter of 25 millimetersand a cutting angle of 160, a kilogram force will produce a maximumsurface score depth of 2.5 millimeters with no significant surfacedefects if the wheel is moved at more than 20 centimeters per second.With a high friction cutting wheel having a diameter of 32 millimetersand a cutting angle of 160, a force of 105 kilograms will produce amaximum surface score depth of 3.1 millimeters without any significantsurface defects if the wheel is moved at more than centimeters persecond. In each of these cases, increasing the applied force beyond thestated maximums creates surface defects that may be removed only withsignificant amounts of seaming, without any increase in score depth.Note that each example states that the wheel speed should be at leastabout 20 centimeters per second. At speeds less than this, subsurfacescores are produced. This probably occurs because the abrasive forces onthe glass are less at lower wheel speeds.

These results seem to indicate that the diameter of the cutting wheeland the force applied thereto should be increased indefinitely. It is tobe noted, however, that with surface scoring, as the diameter of thewheel and the force applied thereto are increased, the length of thewings also increases. This increases the amount of seaming necessary tofinish the edge. Ordinarily, it is not practical to have to scam morethan about 3 millimeters or perhaps, in extreme cases, 6 millimeters ina direction that is transverse to the score. Using a cuttingwheel-having a diameter of 32 millimeters with a cutting angle of 160 itis necessary to seam about 3 millimeters from the edge. This is themaximum amount practical.

If a low friction cutting wheel 14" having a 165 cutting angle and adiameter of 12.7 millimeters applies a force of l l 5 kilograms to apiece of glass 19 millimeters in thickness, a subsurface crack isproduced that begins approximately 0.01 millimeter from the top surfaceand extends for approximately 2.0 millimeters into the thickness of theglass if the wheel is moved at less than about 1 meter per second. Asthe force is increased, significant surface defects develop without anyincrease in crack depth. If a low friction wheel having a diameter of 19millimeters and a cutting angle of 165 applies a force of 175 kilogramsto a piece of glass 19 millimeters in thickness, it is possible toproduce a subsurface crack that starts approximately 0.01 millimeterfrom the glass surface and extends for approximately 2.5 millimeters ifthe wheel is moved at less than about 1 meter per second. If a lowfriction wheel having a diameter of 50 millimeters and a cutting angleof 165 applies a force of 275 kilograms to a piece of glass 1 inch inthickness, a subsurface crack is created that begins approximately 0.01millimeter from the glass surface and extends for approximately 3millimeters if the wheel is moved at less than about 1.6 meters persecond. With low friction wheels, surface deep scores and sometimesspotty subsurface discontinuities are obtained at speeds above thosestated.

As in the case of surface deep scoring, these results seem to indicatethat the diameter of the low friction wheel 14" and the force appliedthereto should be increased indefinitely. In this case, unlike surfacescoring, it is true. With subsurface cracks, as the scoring wheeldiameter increases, it is possible to increase subsurface crack depth,without creating long wings that would necessitate excessive seaming.There does not appear to be any limit, other than the fact that thecrack itself must be seamed, and the greater its depth, the more seamingthat will be necessary. This is easier, however, than seaming lateralwings.

To summarize, low friction wheel 14" having a cutting angle offromapproximately 155 to approximately 170, and preferably 165, and adiameter of at least approximately 12 millimeters, and preferablybetween approximately 18 and millimeters, may be used to produce asubsurface crack with forces of between approximately 80 andapproximately 460 kilograms (or 2 to 3 times surface deep scoringforces). With cutting wheels having cutting angles between and it ispossible to produce both surface and subsurface deep discontinuities byvarying either the force that is applied to the cutting wheel, the speedwith which the cutting wheel is advanced, or the surface finish of thecutting wheel.

Using a cutting wheel with a perfectly blunt cutting angle (i.e., theglass being worked upon is in compression throughout its thicknessbeneath the cutting wheel. lf the cutting angle be reduced, the glasswill no longer be in compression throughout its entire thickness beneaththe wheel, but rather, a tension zone will be created adjacent to thesurface of the glass that is opposite the surface being scored. It isknown that glass fails more easily in tension than in compression. For acutting wheel having a given cutting angle, such as 160, the location ofthe tension zone (which corresponds to the location of thediscontinuity) may be moved by varying the force that is applied to thecutting wheel. For example, if a high friction cutting wheel has adiameter of 19 millimeters and a cutting angle of 160, it may be used toapply either a surface deep score or a subsurface deep score in a pieceof flat glass that is 19 millimeters thick. If a force of approximately80 kilograms is applied to said wheel, a zone of tension is createdadjacent to the top surface of the glass, and a surface deep score willbe created at speeds greater than about 25 centimeters per second. Atspeeds below this, it is likely that a subsurface score will result. Ifa force of approximately 120 kilograms is applied to the same wheel, thezone of tension is further beneath the glass surface and a surface deepscore will result only at wheel speeds in excess of about 30 centimetersper second. This illustrates that the force applied to a cutting wheeland the speed with which it is moved may determine whether a surface ora subsurface deep score results.

The exterior surface of the cutting wheel should preferably be finishedso that it has at least a No. 10 finish. 1f the surface of the wheel istoo rough, local stresses may be created in the glass, thereby damagingits surface.

1f the glass is supported on a longitudinally extending knife edgedirectly beneath the intended path of the subsurface discontinuityduring the scoring operation, tension within the glass is increased andit becomes easier to create the subsurface discontinuity. It isimportant that the knife edge be located exactly beneath the intendedpath of the subsurface discontinuity 10 or the tensile stresses aboutthe path will not be uniform. It is impractical to align a knife edgewith the intended path, and often a narrow plate is used, such as analuminum plate approximately 12.7 millimeters in width. This does notprovide tensile stresses within the glass of the same magnitude as thoseproduced with a knife edge, but it is relatively simple to align theplate with the intended path of the subsurface discontinuity and thetensile stresses obtained with a narrow plate are often sufficient.

Generally, the depth of a score or crack, whether surapparent to oneskilled in the art that it is not limited face or subsurface, should besuch as to weaken the t C F x mp the invention may be practiced glasssufficiently so that a bending moment applied in a wareroom to cut acontinuous ribbon of glass, eiabout the score or crack will cause theglass to fracture ther longitudinally or transversely.

with a resultant edge that is smooth, strong, straight 5 Having nowfully disclosed the invention, what we and perpendicular to the majorsurfaces of the piece of claim is as follows:

glass. Table A shows the preferred minimum depths of l. A method ofproducing a subsurface discontinuity scores or cracks for various glassthicknesses. in a piece of glass comprising the steps of:

Table A positioning a wheel that has a diameter of at leastapproximately 12 millimeters on a major surface of a Preferred MinimumDepth g of glass Glass Thickness, of score or Crack, applying a force ofat least approximately 80 kilomllllmcters grams to said wheel in adirection substantially per- 6 pendicular to said major surface. and :3l5 advancing said wheel along the intended path to cre- 25 ate anindentation in said surface and, concurrently, a subsurfacediscontinuity therebelow in the glass.

Referring I0 Table there IS ShOWH the ranges of 2. A method of producinga subsurface discontinuity force that y be applied to l65cUtting Wheelsof as defined in claim 1, wherein said wheel has an inteious diameters,and the depths of subsurface discontih f at h center th f nd said shaftis nuities that result. The table also indicates the approximounted i bi to reduce f i ti between the mate maximum speeds with which a highfriction wheel h l d i h 1d and a 10W frlfltlon may be advanced to 3. Amethod of producing a subsurface discontinuity Insure that thedlscontmulty Subsurfaceas defined in claim 1, wherein said discontinuityis es- Table B Range of Depth Maximum Speed Maximum Speed of subsurfacefor Producing for Producing Wheel Diameter, Range of Force,Discontinuity, Subsurface Score Subsurface Crack millimeters kilogramsmillimeters (centimeters/second) (meters/second) 12.7 80- 115 l.52.() 25l 19 l l75 l.5-2.5 25 l 31 I80 200 1.7 2.7 30 1.2 50 200 275 2.0 3.0 L6I00 300 460 2.5 4.0 50 2.0

It is anticipated that the present invention may be tablished in a pathsubstantially closer to one of said used to cut edges other thanstraight edges. Further, surfaces than to the other of said surfaces.bent or other forms of flat glass may also be cut as 40 4. A method ofproducing a subsurface discontinuity herein contemplated. ln addition,the invention may as defined in claim 1, further including the step of:

also be practiced in cutting glass objects such as thick placing asurface defect in the glass at the beginning cylinders, rods and tubes.of said intended path before said discontinuity is While the inventionhas thus far been described in produced to initiate said discontinuity.

connection with cutting pieces of flat glass, it will be UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3.865.294 DatedFebruarv 11. 1975 Inventor(s) Fred M. Ernsberger et 211 It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 5, line 14, "29" should be --l9--.

Signed and sealed this 29th day of April 1975.

(SEAL) Attest C. MARSHALL DANN RUTH C. MASON Commissioner of PatentsAttesting Officer and Trademarks FORM PO-IOSO (10-69) u c -pc qog7 -p59I ":S. GOVIIIIIIT PRINTING OI'IICI lul ll-lll,

1. A method of producing a subsurface discontinuity in a piece of glasscomprising the steps of: positioning a wheel that has a diameter of atleast approximately 12 millimeters on a major surface of a piece ofglass, applying a force of at least approximately 80 kilograms to saidwheel in a direction substantially perpendicular to said major surface,and advancing said wheel along the intended path to create anindentation in said surface and, concurrently, a subsurfacediscontinuity therebelow in the glass.
 2. A method of producing asubsurface discontinuity as defined in claim 1, wherein said wheel hasan integral shaft at the center thereof and said shaft is mounted inbearings to reduce friction between the wheel and its holder.
 3. Amethod of producing a subsurface discontinuity as defined in claim 1,wherein said discontinuity is established in a path substantially closerto one of said surfaces than to the other of said surfaces.
 4. A methodof producing a subsurface discontinuity as defined in claim 1, furtherincluding the step of: placing a surface defect in the glass at thebeginning of said intended path before said discontinuity is produced toinitiate said discontinuity.